1. Field of the Invention
The present invention relates to an apparatus and method for transmitting/receiving system information in a wireless communication system with a hierarchical cell structure. More particularly, the present invention relates to an apparatus and method in which, in a wireless communication system with a hierarchical cell structure in which a macro cell and a micro cell using the same frequency band are mixed, a Mobile Station (MS) within the micro cell efficiently acquires, despite mutual interference, system information on the macro cell and system information on the micro cell.
2. Description of the Related Art
In a cellular wireless communication system, a channel state may be deteriorated between a Mobile Station (MS) and a Base Station (BS) due to geography, distribution within a cell, a distance between the MS and the BS, and other reasons. The deterioration of the channel state results in signal attenuation and thus communication between the MS and the BS is not smoothly performed. For example, even within a service area of a macro BS, a building may cause a propagation shadow region. If an MS is located in the propagation shadow region, the macro BS fails to perform smooth communication with the MS. Also, if there are commercial districts or public facilities of dense population within the service area of the macro BS, the deterioration of the service may take place. Such conditions make smooth communication between the macro BS and a plurality of MSs very difficult.
In order to address the service problem of the propagation shadow region while increasing the service capacity and providing a high speed data service, the wireless communication system provides a micro cell service such as a hot zone within the coverage area of a macro BS. Because the hot zone is provided in a public place and provides service to a plurality of users, the hot zone acts as interference in the service area of the macro BS. Here, the wireless communication system in which a macro cell and the hot zone are mixed is of a hierarchical cell structure as illustrated in FIG. 1 below.
FIG. 1 is a diagram illustrating a hierarchical cell structure of a wireless communication system in which a macro cell and a hot zone are mixed according to the conventional art.
As illustrated in FIG. 1, the wireless communication system includes a macro BS 101 managing a macro cell 110, and hot zone BSs 102, 103, and 104 managing respective hot zones 120, 130, and 140.
Here, the hot zone BSs 102, 103, and 104 and the macro BS 101 use the same frequency band for the sake of the efficient use of frequency resources. If so, a service of the macro BS 101 is greatly affected by interference at a boundary between service areas of the hot zone BSs 102, 103, and 104 and the macro BS 101. Thus, an MS fails to smoothly receive a signal from the macro BS 101. In a wireless communication system having no hot zones, if interference takes place at a boundary between the service area of the macro BS 101 and service areas of neighbor macro BSs, the wireless communication system applies cell planning, sectorization, etc. between the macro BS 101 and the neighbor macro BSs, thus being able to reduce interference between neighbor cells within system requirements. However, if a large number of existing hot zones (e.g., 120, 130, and 140) are included in the macro cell 110, it is difficult to control interference through the cell planning.
On the other hand, if an MS enters a service area of a neighbor BS out of a service area of a serving BS from which the MS receives a service, the MS performs handover to the neighbor BS to receive a seamless service. In the above process, the MS sends and receives a handover message with the serving BS and the neighbor BS (i.e., a target BS) that is intended for handover.
A process of initiating handover according to a request of an MS in an IEEE 802.16e wireless communication system according to the conventional art is described below. An MS periodically measures strengths of preamble signals received from a serving BS and neighbor BSs. At this time, if the signal strength of the serving BS is less than a threshold strength and the signal strength of the neighbor BS is more than a threshold strength, the MS determines the neighbor BS as a target BS and sends a MOBile HandOver REQuest (MOB_HO-REQ) message to the serving BS. Then, upon receiving the handover request message, the serving BS sends a HandOver notification (HO-notification) message to the target BS through a backbone network. Upon receiving the HO-notification message, the target BS sends a HandOver notification ReSPonse (HO-notification-RSP) message to the serving BS and previously prepares a handover process. At this time, the serving BS sends the MS a MOBile HandOver ReSPonse (MOB_HO-RSP) message in reply to the handover request. Upon receipt of the response, the MS determines to perform handover and sends a MOBile HandOver INDication (MOB_HO-IND) message to the serving BS. Then, the MS initiates an operation of handover to the target BS.
In a wireless communication system constituted of only macro cells, the handover process makes it possible to communicate with no interruption even at a boundary between the macro cells. However, in a wireless communication system with a hierarchical cell structure in which a macro cell and one or more hot zones are mixed, a problem taking place at a boundary between cells may not be addressed only through handover. Its causes are provided as follows. First, because there are a great number of hot zones within a macro cell, an MS performs many handover processes according to movement even within a service area of a macro BS. Thus, a large number of the aforementioned handover related messages are exchanged between an MS and a serving BS or target BS. The large number of messages sent results in a problem of reducing the whole system capacity. Second, there is a problem that the coverage of a hot zone is small. The hot zone provides a service coverage area having a radius of 5 meters to 50 meters. If an MS passes the hot zone at high speed, because a passing time within the hot zone is shorter than a handover performance time, the MS has to seamlessly perform handover to a different target BS during handover to the hot zone or soon after handover to the hot zone. Thus, there is a need for a scheme in which, although entering the coverage of a hot zone, an MS moving at high speed may perform communication not with the hot zone but with the macro cell.
If an MS performs communication with a macro cell within a hot zone as above, a BS may allocate a channel resource that causes less interference from the hot zone through efficient scheduling for the sake of data transmission, and enable data retransmission through a Hybrid Automatic Repeat request (HARQ) technique. Thus, in the case of data, it is possible to communicate without interruption even at a boundary between cells. However, in the case of a common control channel such as a preamble signal, system information, etc., it is impossible to alleviate interference through scheduling because a resource region for transmission is previously defined. Also, if the preamble signal, the system information, etc. are not received during a predefined period of time, communication between a BS and an MS is interrupted. Thus, there is a need for a scheme in which, if an MS performs communication with a macro cell within a hot zone, the MS may efficiently acquire the preamble signal, the system information, etc.
As described above, a hot zone provided in a public place may cause significant interference in a service area of a macro BS and give rise to an interruption of a communication service, further resulting in a decrease of a system capacity. Thus, there is a need for an apparatus and method for effectively preventing conditions in which a service of a macro BS is stopped because of interference from a hot zone.